1. Field of the Invention
The present invention relates to a polarizing element in which a birefringence phenomenon is exploited, such as a Wollaston prism, a Glan-Thompson prism, or the like.
2. Description of the Related Art
It has conventionally been known that, when unpolarized light is incident on an anisotropic crystalline substance, such as calcite (CaCO3), quartz (SiO2), rutile (TiO2), lithiumniobate (LiNbO3), or the like, two pencils of refractive light rays are observed. This is called a birefringence phenomenon, and the two refractive light rays are called a normal light ray which is obedient to Snell""s law and an abnormal light ray which is disobedient to Snell""s law, respectively.
Here, the greater the index of birefringence, i.e., the difference in refractive index between the normal and abnormal light rays, the more notable the birefringence derived from light incident in a direction nonparallel to an optical axis. This results in an increase in the separation angle between the normal and abnormal light rays.
Accordingly, if the separation angle between the normal and abnormal light rays is assumed to be kept constant, by employing a crystal having as great a birefringence index as possible, it is possible to realize an element having a shorter length. Reduction in the size of an element contributes to the miniaturization of apparatuses necessitating a polarizing element, such as the light-reading portion of an optical recording apparatus, an optical isolator, or the like. Particularly, in recent years, as larger and larger capacities are achieved in optical recording apparatuses, the light source wavelengths have come to be made shorter and shorter. For example, a blue-color semiconductor laser having a wavelength of about 400 nm is expected to be used as a light source.
However, conventionally-used crystal materials have the following disadvantages. The optical data of typical birefringent crystal materials will be shown in Table 1. Note that, in the table, symbols no, ne, and xcex94n represent the refractive index of a normal light ray, the refractive index of an abnormal light ray, and the birefringence index, respectively, as observed at a wavelength of 633 nm.
For example, as for quartz (SiO2), on the positive side, it is a naturally occurring substance and can nevertheless be produced synthetically by the hydrothermal synthesis method. On the negative side, the birefringence index of quartz is about 0.01, which is unduly small compared to other crystals, and therefore its use leads to an unduly long element length. The quartz is a material which exhibits satisfactory transmittability not only for blue-color light but also for ultraviolet light.
As for calcite (CaCO3), on the positive side, it has a birefringence index of about 0.16, which is sufficiently large compared to other crystals, and exhibits satisfactory blue-light transmittability. On the negative side, calcite occurs naturally only in nature. This makes it difficult to provide an inexpensive and high-quality element.
Moreover, as for a rutile (TiO2) single crystal, on the positive side, it has an extremely large birefringence index of about 0.29 and is thus suitably used in a polarizer designed for an optical isolator or the like at present. On the negative side, rutile can be developed only by the Verneuil""s method or floating zone method. The difficulty of obtaining a substance with satisfactory crystallzability causes a problem in that a reduction in yields occurs due to the existence of grains and inner warps, or a large-sized crystal cannot be obtained easily. For this reason, it is inevitable that a rutile-made element becomes expensive. A slight yellowish tint is recognized in rutile even under visual observations. From this fact, it is understood that rutile offers poor blue-light transmittability.
Further, a lithium niobate (LiNbO3) single crystal has in recent years attracted attention. This is because, such a large-size crystal as has a crystal diameter of 0.076 to 0.102 m can be developed relatively easy by the Czochralski method. However, lithium niobate has the following disadvantage. For example, just like a process for fabricating a polarizing prism, in a case where a crystal is cut up at a predetermined angle with respect to the C axis and then the polished crystal portions are bonded together, if, as the crystal material, lithium niobate or rutile having a larger refractive index (equal to 2.0 or above) is used, proper adhesive is unavailable that has a refractive index close to that of the material. Therefore, it is essential to suppress reflection occurring in the bonded area and the loss of transmission light by providing, in accordance with the transmission wavelength, a reflection prevention film, such as a dielectric substance, for adhesive in use. This process makes the manufacture complicated, and consequently the finished element becomes expensive.
Note that, to improve the stability of an optical system, in some instances, a plurality of prisms are bonded together to form a single unit of a combined prism. In connection with this, in a case where optical components made of materials having larger refractive indices than in an ordinary glass material are integrated together, similar problems arise. Moreover, lithium niobate offers poor blue-light transmittability.
Yttrium vanadate (YVO4) is a positive uniaxial crystal which is developed by the Czochralski method. It offers a significantly large birefringence index of about 0.2 or above in a range from the visible region to the near-infrared region. Yttrium vanadate is mechanically and physically excellent and is thus frequently used as a substitute for calcite or rutile to form an optical polarizer. However, the absolute value of the refractive index of Yttrium vanadate is so large that it has similar disadvantages to those of lithium niobate and rutile from the standpoint of selecting materials including adhesive and combined optical components. Moreover, the blue-light transmittability is poor.
As for lithium tetraborate (Li2B4O7), its single crystal is synthesized by the Czochralski method or other means. Lithium tetraborate has a refractive index substantially the same as that of a normal glass or plastic, and offers satisfactory blue-light transmittability. However, as compared with the above stated Yttrium vanadate and rutile, the birefringence index is small. Since lithium tetraborate is a crystal material, the cost cannot be reduced easily.
FIG. 6 is a perspective view showing an example of conventional wedge-like elements. Japanese Unexamined Patent Publication JP-A 6-59125 (1994) proposes a method of using a liquid crystal polymer instead of the above stated crystal materials. The example shown in the figure is a beam splitter 105, an example of a wedge-like element formed of a unidirectionally oriented polymer material composed of a setting liquid crystal monomer composition. A manufacturing method therefor will be described below.
Firstly, two rectangular glass plates are coated with a nylon-made orientation layer and are then rubbed with a soft pile cloth or the like in a selected direction parallel to one end of each glass plate. Secondly, the two glass plates are arranged face to face with each other with a wedge-like space secured therebetween such that the rubbing direction of the two glass plates is aligned with a parallel direction. Then, a liquid crystal monomer is charged into the wedge-like space. Lastly, UV-irradiation treatment is conducted to form a solid wedge-like element, and subsequently the glass plates are removed. As a result, the polarized light sensitivity beam splitter 105 shown in FIG. 6 is realized. As the filler monomer, a liquid crystal diacrylate compound to which a photosensitive initiator is added is desirable.
Instead of the nylon orientation layer, it is also possible to use a rubbed layer made of polyimide or polyethylene, a silicon oxide layer sputtered at a certain angle, or the like. Moreover, magnetic field orientation may be adopted as required.
The above stated method of using a crystal material is at a disadvantage in that a liquid crystal material is expensive and offers poor blue-light transmittability, that the absolute value of the refractive index exceeds 2.0 and thus it is impossible to bring the crystal material into conformity with other materials, and that the birefringence index is unduly small.
On the other hand, the above stated method of using a liquid crystal polymer is at a disadvantage in that it requires some time for detaching and reattaching glass plates, which results in an undesirable increase in the number of manufacturing steps and thus the cost. In addition, the effect of an orientation layer generally reaches only to a location several xcexcm thickness distance away from the orientation layer, and the distribution of the degree of orientation is indicated in accordance with a distance from the orientation layer. Thus, it is difficult to fabricate a homogeneous birefringent component with a thickness of several tens to several hundreds xcexcm.
An object of the invention is to provide a polarizing element which has a sufficiently large birefringence index and exhibits satisfactory blue-light transmittability by a simple and inexpensive method for manufacturing a polarizing element.
The invention provides a polarizing element comprising:
at least one optical block shaped like a substantially triangle pole,
wherein the optical block comprises a high polymer film formed by stretching an organic material which exhibits light transmittability and birefringence.
According to the invention, by employing an organic material which exhibits light transmittability and birefringence, a polarizing element can be provided at lower cost compared to the case where an expensive crystal material is used. Moreover, by composing the optical block of a high polymer film formed by stretching an organic material, a homogeneous polarizing element can be provided in which the birefringence index is arbitrarily adjusted in accordance with the degree of stretching.
In the invention, it is preferable that, in cases where the polarizing element comprises two or more optical blocks, these optical blocks are fabricated into a layered structure, and any two optical blocks contiguous to each other of the optical blocks have mutually different optical axes.
According to the invention, by laminating two optical blocks contiguous to each other such that their optical axes are mutually incoincident, it is possible to use a larger number of optical blocks which are to be layer-structured and thereby secure a larger separation angle between individual polarized light beams. This helps suppress adverse effects, such as scattered light, and improve the light-extinction ratio.
In the invention, it is preferable that the high polymer film is made of polyethylene terephthalate.
According to the invention, by employing polyethylene terephthalate offering satisfactory blue-light transmittability, it is possible to provide a polarizing element applicable to an apparatus in which a blue-light semiconductor laser is used as a light source. Moreover, the value of the refractive index is, for example, at a wavelength of 780 nm, about 1.7 in a stretch direction and about 1.5 in an orthogonal direction. This makes it easy to bring the material into conformity with other materials such as adhesives.
In the invention, it is preferable that the polarizing element comprising the optical blocks has a thickness of several tens to several hundreds xcexcm in a direction in which incident light passes therethrough.
According to the invention, even if the optical block formed by stretching an organic material or the optical block in question taking on a layered structure is made to have a thickness of several tens to several hundreds pm, since the high polymer film is stretched evenly, it is possible to provide a thick-walled polarizing element having an even birefringence index.
The invention provides a method for manufacturing a polarizing element comprising the steps of:
forming a prism sheet of a high polymer film by stretch-processing an organic material which exhibits light transmittability and birefringence; and
cutting up the formed prism sheet to obtain an optical block having a predetermined shape.
According to the invention, by composing the polarizing element of an optical block obtained by cutting up a stretch-processed high polymer film, an inexpensive, homogeneous polarizing element can be fabricated with a simple step.
In the invention, it is preferable that, in the sheet forming step, a plurality of prism sheets are formed and the formed plurality of prism sheets are laminated on one another such that any two sheets contiguous to each other of the prism sheets have mutually different optical axes, thereby forming a layer-structured prism sheet, and that, in the sheet cutting step, a layered structure of optical blocks having a predetermined shape is obtained by cutting.
According to the invention, by cutting a plurality of polarizing elements from a layered structure of a plurality of prism sheets laminated on one another, polarizing elements can be manufactured in large quantities at lower cost with a simple step, i.e., without the need to make an adjustment to the optical axes of the optical blocks on a polarizing element-by-polarizing element basis.
In the invention, it is preferable that, in the sheet forming step, the organic material is stretched to an arbitrary length.
According to the invention, by stretching the organic material to an arbitrary length, the birefringence index of the high polymer film thus obtained can be changed in accordance with the degree of the stretching. This makes it possible to fabricate a polarizing element having the desired birefringence index.
As described heretofore, according to the invention, by a simple and inexpensive method for manufacturing a polarizing element using a high polymer film, such as PET, formed by stretching an organic material which exhibits light transmittability and birefringence, it is possible to provide a polarizing element having a sufficiently large birefringence index and offering satisfactory blue-light transmittability.